The present invention relates to a vacuum furnace for heat treatment of workpieces and materials.
In general, vacuum furnaces are used for melting or brazing of a metal workpiece such as an aluminum workpiece. This is primarily due to the fact that the metal may be oxidized by atmospheric oxygen with the workpiece itself or its junction being deteriorated as a consequence. As shown in FIG. 1, a typical example of the vacuum furnace available for this purpose is constructed from horizontally extending, cylindrical bodies, viz., a furnace proper 1, a pre-exhaust chamber 3 adapted to feed workpieces therethrough and put in communication within the furnace proper 1 through a partitioning vacuum valve 2, and a pre-exhaust chamber 4 adapted to discharge the workpieces therefrom and put in communication within the furnace proper 1 through a partitioning vacuum valve 2'. Within and across the furnace proper 1, there is an line assembly 5 for delivering the workpieces.
During the treatment of a workpiece A, both valves 2 and 2' are closed to maintain the furnace proper 1 independent and the chambers 3, 4 in an air-tight condition, whilst the furnace proper 1 is maintained in a given vacuum condition and preheated to a suitable temperature in normal state. After a door 6 is opened to deliver the workpiece A into the pre-exhaust chamber 3 and closed, the chamber 3 is evacuated to a degree of vacuum substantially equal to that prevailing in the furnace proper 1. The vacuum valve 2 is then opened to communicate the pre-exhaust chamber 3 with the furnace proper 1. Thereupon, the workpiece A is fed from the chamber 3 into the furnace proper 1, and the vacuum valve 2 is closed to cut off communication between the furnace proper 1 and the pre-exhaust chamber 3. Subsequently, the furnace proper 1 is brought to a desired high temperature under the given vacuum to work or treat the workpiece A. After the given heat treatment is completed, the vacuum valve 2' is opened to communicate the furnace proper 1 with the pre-exhaust chamber 4 exhausted in advance to the degree of vacuum prevailing in the furnace proper 1. The workpiece A is then fed from the furnace proper 1 into the pre-exhaust chamber 4 by means of the delivery line assembly 5. After the communication between the furnace proper 1 and the pre-exhaust chamber 4 is cut off by manipulation of the valve 2', the chamber is brought to an atmospheric pressure, and the workpiece A is carried out from the arrangement by opening a door 7.
In the arrangement as referred to above, the heating furnace proper 1 is arranged on a level with the pre-exhaust chamber 3 and 4. As a result, the partitioning vacuum valves 2 and 2', which are also arranged on a level with the furnace proper 1, are exposed directly to radiant heat, and heated while the furnace is operated. In this connection, it is noted that convection poses no problem under vacuum. As a result, the valves may be subject to deformation, or their sliding parts may lose their lubricating properties, so that difficulties are encountered both in closing and opening thereof and in the keeping of air-tightness. There may also be certain deformation of the delivery line assembly such as a conveyor or chain block assembly for delivery of a workpiece A from the chamber 3 into the chamber 4 through the furnace proper 1. This interferes with the running of the furnace from a practical point of view, and becomes increasingly marked, especially when the furnace is of a larger size.